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Helical plant viruses are arranged into four families: Flexiviridae, Tobamoviridae, Hordeiviridae, and Tobraviridae. Further division of these families into genera and subgenera is shown. A family-specific, constant amount of RNA per unit virion length and a distinct radial location of RNA from the center of the helix are considered as basic criteria for establishment of these families.
The spread of plant virus disease between fields is reviewed for several horticultural and agricultural crops. Previous studies have focused on cropping systems where there is scope for using separation in time and space to reduce the potential for virus spread between plantings. In our study, data are presented on virus disease spread between fields in an irrigated rice area in the tropics where crops are grown continuously throughout the year. An intensive survey was conducted in rice fields planted from November 1992 to October 1994 in which the occurrence of new infections of rice tungro virus disease was recorded. The effect of a number of variables on disease incidence, including the proximity of a field to inoculum sources, was examined using ordinal regression analysis. Primary infection showed large seasonal fluctuation. In addition, the number of leafhopper vectors had a significant effect, as did the tungro vector resistance of the rice variety grown. The distance to nearest inoculum source and the tungro incidence of this source significantly affected the level of infection occurring in a vulnerable field. The results are discussed in relation to management strategies to reduce the spread of tungro disease between fields in irrigated lowland rice cropping systems.
Most plant viruses are absolutely dependent on a vector for plant-to-plant spread. Although a number of different types of organisms are vectors for different plant viruses, phloem-feeding Hemipterans are the most common and transmit the great majority of plant viruses. The complex and specific interactions between Hemipteran vectors and the viruses they transmit have been studied intensely, and two general strategies, the capsid and helper strategies, are recognized. Both strategies are found for plant viruses that are transmitted by aphids in a nonpersistent manner. Evidence suggests that these strategies are found also for viruses transmitted in a semipersistent manner. Recent applications of molecular and cell biology techniques have helped to elucidate the mechanisms underlying the vector transmission of several plant viruses. This review examines the fundamental contributions and recent developments in this area.
To avoid destruction in the haemolymph of their vector, many plant circulative viruses interact with GroEL homologues produced by insect endosymbiotic bacteria. We have exploited this phenomenon to devise tools allowing trapping of plant viruses by either GroEL purified from the whitefly Bemisia tabaci or by whitefly GroEL over-expressed in E. coli. PCR tubes or 96-well plates coated with a GroEL preparation were incubated with cleared sap of virus infected plant leaves or insect vectors. GroEL-bound viruses were then identified by PCR or RT-PCR using virus-specific primers or by ELISA with virus specific antibodies. In this way Tomato yellow leaf curl virus (TYLCV) - a whitefly-transmitted geminivirus - was detected in plant sap, in extracts of leaf squashes and in homogenates of individual viruliferous whiteflies. Anti-GroEL antibody prevented TYLCV binding to GroEL. GroEL-bound virus was also detected by ELISA. GroEL was much more potent in binding TYLCV than commercial anti-TYLCV antibodies. In addition to several other geminiviruses, these procedures allowed detecting a variety of RNA viruses such as Cucumber mosaic virus (CMV), Prune dwarf virus (PDV) and Tomato spotted wilt (TSWV), but not Potato virus X and Potato virus Y (PVX and PVY), Grapevine leafroll-associated viruses (GLRV) and Tobacco mosaic virus (TMV). Predictions pertaining to viruses that do, or do not bind to GroEL, and applications in plant virus diagnosis, are presented.
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Cowpea chlorotic mottle virus RNA has been prepared in comparatively high yield (at least 50%) by a modified phenol extraction method. The preparation, which has high biological activity, has been resolved into four components by zonal centrifugation on a 15-40% (w/v) sucrose density gradient. The components obtained have been tested for biological activity against whole plants and plant protoplasts. Each of the two largest components RNA 1 and RNA 2 was by itself infective (50-90% of the specific infectivity of the whole genome) and produced virus-specific proteins (coat protein and P2) and RNAs ("RNA 3" and "RNA 4"). Contamination by small proportions (less than 10%) of neighbouring RNAs is presumed to be involved in this infectivity. The two smallest components were obtained in an almost pure form.
▪ Abstract Plant viruses have had an impact on the science of virology and on plant pathology ever since the virus concept was discovered with Tobacco mosaic virus at the end of the nineteenth century. In this review, we highlight those discoveries. We have divided plant virus research into a "Classical Discovery Period" from 1883-1951 in which the findings were very descriptive; an "Early Molecular Era" from 1952 to about 1983, in which information was developed that described further properties of the viruses, aided by the development of a number of salient techniques; and the "Recent Period" from 1983 to the present, when techniques have been developed to modify plant virus genomes, to detect nonstructural gene products, to determine the functions of viral gene products, and to transform plants to elicit novel forms of resistance to viral diseases. In this period, plant virology has played a significant role in formulating an understanding of the mechanisms of gene silencing and recombination, plasmodesmatal function, systemic acquired resistance, and in developing methods for pathogen detection. We also attempt to predict the direction plant virology will take in the future.
Many plant virus genera encode a 'triple gene block' (TGB), a specialized evolutionarily conserved gene module involved in the cell-to-cell and long-distance movement of viruses. The TGB-based transport system exploits the co-ordinated action of three polypeptides to deliver viral genomes to plasmodesmata and to accomplish virus entry into neighbouring cells. Although data obtained on both the TGB and well-studied single protein transport systems clearly demonstrate that plant viruses employ host cell pathways for intra- and intercellular trafficking of genomic nucleic acids and proteins, there is no integral picture of the details of molecular events during TGB-mediated virus movement. Undoubtedly, understanding the molecular basis of the concerted action of TGB-encoded proteins in transporting viral genomes from cell to cell should provide new insights into the general principles of movement protein function. This review describes the structure, phylogeny and expression of TGB proteins, their roles in virus cell-to-cell movement and potential influence on host antiviral defences.
A viable strain of cauliflower mosaic virus is described which arose by illegitimate recombination of two lethal parents. In this strain, the normally overlapping open reading frames IV and V, corresponding to the retrovirus gag and pol genes, are separated by a short intergenic region, suggesting that in this virus and in contrast to retroviruses, fusion of gag and pol gene products is not obligatory.
A simple and rapid procedure of enzyme immunoassay (PTA-ELISA) was used to detect and identify viruses in individual plants. Virus antigen in crude leaf extracts was adsorbed directly to a solid-phase support, allowed to react with unfractionated antiserum and the antigen-antibody complex detected with a general purpose conjugate of protein A and enzyme. Viral antigens were trapped most effectively by high bonding polystyrene microtitre plates loaded with leaf extracts prepared in carbonate buffer at pH 9.6. With protein A-alkaline phosphatase conjugate and the substrate p-nitrophenyl phosphate as the antibody-detection system, 18 plant viruses in 8 virus groups were detected reliably and nonspecific reactions did not occur. However, when the substrate 3,3',5,-tetramethyl benzidine was used in conjunction with protein A-horseradish peroxidase conjugate, nonspecific reactions were given by leaf extracts from some uninfected or virus-infected plant species. Where less sensitivity is required than is provided by versions of ELISA that rely on antibody-captured antigen, this method provides a simple and rapid means of detecting and identifying viruses in crude sap extracts with the aid of unfractionated antisera.
The plant virus cowpea mosaic virus (CPMV) has recently been developed as a biomolecular platform to display heterologous peptide sequences. Such CPMV-peptide chimeras can be easily and inexpensively produced in large quantities from experimentally infected plants. This study utilized the CPMV chimera platform to create an antiviral against measles virus (MV) by displaying a peptide known to inhibit MV infection. This peptide sequence corresponds to a portion of the MV binding site on the human MV receptor CD46. The CPMV-CD46 chimera efficiently inhibited MV infection of HeLa cells in vitro, while wild-type CPMV did not. Furthermore, CPMV-CD46 protected mice from mortality induced by an intracranial challenge with MV. Our results indicate that the inhibitory CD46 peptide expressed on the surface of CPMV retains virus-binding activity and is capable of inhibiting viral entry both in vitro and in vivo. The CD46 peptide presented in the context of CPMV is also up to 100-fold more effective than the soluble CD46 peptide at inhibiting MV infection in vitro. To our knowledge, this study represents the first utilization of a plant virus chimera as an antiviral agent.
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Genetic resistance to plant viruses has been used for at least 80 years to control agricultural losses to viral diseases. To date, hundreds of naturally occurring genes for resistance to plant viruses have been reported from studies of both monocot and dicot crops, their wild relatives, and the plant model, Arabidopsis. The isolation and characterization of a few of these genes in the past decade have resulted in detailed knowledge of some of the molecules that are critical in determining the outcome of plant viral infection. In this chapter, we have catalogued genes for resistance to plant viruses and have summarized current knowledge regarding their identity and inheritance. Insofar as information is available, the genetic context, genomic organization, mechanisms of resistance and agricultural deployment of plant virus resistance genes are also discussed.
Populations of plant viruses, like all other living beings, are genetically heterogeneous, a property long recognized in plant virology. Only recently have the processes resulting in genetic variation and diversity in virus populations and genetic structure been analyzed quantitatively. The subject of this review is the analysis of genetic variation, its quantification in plant virus populations, and what factors and processes determine the genetic structure of these populations and its temporal change. The high potential for genetic variation in plant viruses, through either mutation or genetic exchange by recombination or reassortment of genomic segments, need not necessarily result in high diversity of virus populations. Selection by factors such as the interaction of the virus with host plants and vectors and random genetic drift may in fact reduce genetic diversity in populations. There is evidence that negative selection results in virus-encoded proteins being not more variable than those of their hosts and vectors. Evidence suggests that small population diversity, and genetic stability, is the rule. Populations of plant viruses often consist of a few genetic variants and many infrequent variants. Their distribution may provide evidence of a population that is undifferentiated, differentiated by factors such as location, host plant, or time, or that fluctuates randomly in composition, depending on the virus.
The development of genetically engineered resistance to plant viruses is a result of efforts to understand the plant-virus interactions involved in "crossprotection," a phenomenon observed with several plant virus diseases. Historically, expression of the coat protein gene of Tobacco mosaic virus in transgenic tobacco (Nicotiana tabacum) plants is the first example of transgene-mediated resistance to a plant virus. Subsequently, virus-derived sequences of several plant viruses were shown to confer virus resistance in experimental and/or natural hosts. For plant RNA viruses, virus complementary DNA sequences shown to confer resistance include wild-type genes, mutated genes that produced truncated protein products, and nontranslatable sense or antisense transcripts to various regions of the virus genome. Resistance also has been demonstrated for some viruses by mutant trans-dominant gene products, derived from the movement protein and replication-associated protein genes. In addition to virus-derived sequences, gene sequences of plant origin have also been used for transgenic resistance, and such resistance can be virus-specific, for instance, R genes isolated from resistant plant genotypes, or nonspecific, for example, ribosome inactivating proteins and proteinase inhibitors. Plantibodies and 2-5A synthetase, a class of proteins of mammalian origin, have also been useful in engineering plant virus resistance. In the case of transgenic resistance mediated by viral coat protein, the mechanism of resistance was suggested to operate during the early events of virus infection. However, transgene-mediated RNA silencing and generation of small interfering RNAs appears to be the primary mechanism that confers resistance to plant viruses. Despite the advantages of transgene-mediated resistance, current interest in the development and use of transgenic virus resistant plants is low in most parts of the world. However, because of its real potential, we believe that this technology will have more widespread and renewed interest in the near future.
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Plants viruses are versatile vectors that allow the rapid and convenient production of recombinant proteins in plants. Compared with production systems based on transgenic plants, viral vectors are easier to manipulate and recombinant proteins can be produced more quickly and in greater yields. Over the last few years, there has been much interest in the development of plant viruses as vectors for the production of vaccines, either as whole polypeptides or epitopes displayed on the surface of chimeric viral particles. Several viruses have been extensively developed for vaccine production, including tobacco mosaic virus, potato virus X and cowpea mosaic virus. Vaccine candidates have been produced against a range of human and animal diseases, and in many cases have shown immunogenic activity and protection in the face of disease challenge. In this review, we discuss the advantages of plant virus vectors, the development of different viruses as vector systems, and the immunological experiments that have demonstrated the principle of plant virus-derived vaccines.
As with animals, plants are susceptible to viral diseases. Until recently, plant viruses had no direct bearing on animal health, though the crop losses they can cause may have indirect effects. However, recent advances in recombinant DNA technology make it possible to adapt plant viruses so that they can produce pharmaceutically important proteins in plants.